In an electron accelerator, when an electron gun emits electrons, a filament in the electron gun needs to be heated to a certain temperature and maintained at this temperature. The filament needs to be suspended at a negative high voltage to pull electrons out of a cathode.
In a conventional manner, in order to avoid an excessive power of a filament power supply, the voltage of the filament power supply level is increased gradually level by level to heat the filament. For example, when the filament is at a lower temperature, the filament is of a smaller resistance. At this time, the voltage of the filament power supply is adjusted to be at a lower voltage level to heat the filament. When the filament is at a higher temperature, the filament is of a greater resistance. At this time, the voltage of the filament power supply is adjusted to be at a higher voltage level to heat the filament.